A source confirms to E! News Charlie Sheen's ex entered a treatment facility following her hospitalization in November, while her mother, Moira Fiore, is taking care of the former couple's twin sons, Max and Bob, 7.

Entertainment Tonight was the first to break the news as Mueller's sister, Sydney Wolofsky, told the publication the actress was the one who made the decision to go back for treatment and is "doing a lot better."

The news of her rehab comes one month after Mueller was given an evaluation when she went missing with a nanny and her two sons.

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On November 16, the Utah Department of Public Safety received a welfare check call from a woman claiming to be Mueller's mother. Dispatchers responded to the call and located Mueller, her two children and the nanny on Interstate 15 near Nephi, Utah. While Mueller was taken to a hospital for an evaluation, the boys were put in the care of family.

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Now, Mueller's mother is living at the family's house and taking care of the twins "so their daily routine stays the same" while the actress seeks the treatment she needs, Wolofsky told ET.

The sister says they're doing well and will visit their mother on Christmas. "I know that we're going to see her on Christmas. We're going to go over [to the rehab center] to visit and just spend time with her. My brother, his wife and two kids, my parents and I are going to go to Utah to spend Christmas with her," Wolofsky said. "Brooke would not let us show up without the boys."

As for how long Mueller plans to stay in the rehabilitation facility, Wolofsky said she has no plans of leaving until she is 100-percent ready.

She explained, "I know that she doesn't want to leave until she is [there for] a certain amount of time—totally sober with no temptations of any kind, which is really smart on her part."